Display surface for color television tube



May 8, 1956 E. o. LAWRENCE DISPLAY SURFACE FOR COLOR TELEVISION TUBE Filed Dec. 22, 1953 2 Sheets-Sheet 1 IN V EN TOR. ie/wsr 0. zAa/ei/vci 1956 E. o. LAWRENCE 2,745,033

DISPLAY SURFACE FOR COLOR TELEVISION TUBE Filed D60. 22, 1953 2 Sheets-Sheet 2 INVENTOR. ie/viir 0, [Id/Pi/VCE Irma/WK;

2,745,033 DISPLAY SURFACE FORECOLOR TELEVISION TUB Ernest 0. Lawrence, Berkeley, Calif., assignor to Chromatic Television Laboratories, Inc., New York, N. Y., a corporation of California Application December 22, 1953, Serial No.399f753 12 Claims. (Cl. 313-78) This invention relates to cathode-ray tubes for displaying-television images in natural color. Specifically, it relates to display surfaces for tubes of the type wherein three component colors which, additively, produce white light when mixed in appropriate proportions, are displayed in succession so rapidly that the eye views them as if simultaneous. The change from color to color is accomplished by directing a beam of cathode-rays successively to elemental areas of phosphors emissive of the different primary colors chosen, deflection being accom that of the display screen and closely adjacent thereto.

The electrodes of each set are connected, but the two sets are mutually insulated so that alternate electrodes are of the same potential while different potentials may be applied to the two sets. The spacing of adjacent electrodes is of the order of magnitude of one picture element as defined above and the electrodes are sub stantially parallel to the phosphor strips on the screen. The strips are so spaced with relationtov the electrodes that a cathode ray beam, scanning the target area occupied by the display screen and the color controlgrid and centered between any pair of electrodes of the two sets of the grid will also be substantially centered on a phosphor strip emissive of light of one of the three component colors, preferably green; i. e., the stripsjof phosphor emissive of this one color are electroroptically centered behind the mid-point between each pair of impact and excite a phosphor of a single 'color only,

which, in the case proposed, would be green iffthere is no difference of potential between the electrodes of the two sets. If, however, a difference of potential be applied between the two sets of grid electrodes, the elec-'- trons of the beam passing between them will be given a velocity in the direction of the more positive electrode and hence can be made to fall solely on one or the other of the flanking colors. Such constriction can be accomplished by a mask, which can be either part of or sep-- arate from grid electrodes, but preferablyit is-accomplished by utilizing the grid electrodes ascomponents "nited States Patent 2,745,033 Patented May 8, 1956 "ice of electron lenses having, as an additional element, a film or electron permeable layer of conducting material, suchas aluminum, deposited over the phosphor surface.

An electron beam may be developed by a conventional approximately four times that of the color control grid with respect to .the, cathode, all electrons entering between the electrodes of any pair will be focused, in the dimension normal to the length of the grid electrodes, to a narrow line whose length is substantially equal to the diameter of the beam but whose width is only a small fraction thereof, so that practically the entire energy of the beam is concentrated in a small area at the focus.

Tubes of this type can be used to display color television images transmitted by any of the conventional systems, whether the system used be designated as field sequentialf? line sequential, dot sequential or simultaneous, the designation referring to whether information relative to the respective colors is transmitted se-' quentially or simultaneously. 'The system now appearing most likely to be adopted for general use is the socalled N. T. S. C. (National Television Systems Committee) system, which is, in efiect, a simultaneous system in that in the signals received and acted upon by the receiving equipment there is always present a signal representative of the instantaneous intensity of eachtcomponent color constituent of the hue to be displayed.

Since tubes of the character herein considered have only a single electron gun'and only one (or at most two) of the component colors can be displayed at any one instant, simultaneous signals must be broken down into dot sequential signals fordisplay upon the screen. In order to do this therelative potentials of the electrodes composing the grid are alternated rapidly about the mean potential'thereof, so that the beam falls on the colors successively in the order red, green, blue, green, red, etcQ'at a frequency of several million cycles per second.- Over'intervals of one color cycle at this frequency the'eye integrates the light of each component color-over the time during which it is received, so that the apparent intensity which it registers is pro portional not only to the intensity of the electron beam itself but also to the period during which it falls upon any one'color. It follows that if some element of the picture field is white, and is to be reproduced as such upon the television screen, the beam in its oscillation across the strips must impinge upon strips emissive of each color for equal lengths of time, assuming equal phosphor efiiciencies.

The electrical capacity between the two sets of electrodes of the color gridis relatively high, and in order tov achieve the necessary deflection of the beam with a moderate amount of powerin the deflecting circuit, it is practically necessary to connect the sets of electrodes in a resonant oscillating circuit, tuned to the frequency 'of the color deflection. When this is done the relative potential of the two sets'of electrodes varies substantially in accordance with a sine law. Accordingly, the velocity of the spot of illumination produced by the electron beam, as itis deflected by the color control potentials in the direction transverse to the long dimension of the phosphor strips, is constantly varying. To produce the proper color balance the beam should impact a strip of each color through one-third of each cycle of the color sequence, or as nearly as possible, for

out of each 360 of the color cycle. Since the intermediate strip is traversed by the beam twice in each cycle, while the red and biue strips are each traversed but once per cycle, the beam, ideally, should fall on the green strip for 60 electrical degrees out of each half cycle, or 30 electrical degrees on either side of the undeflected position. Since the amount of deflection is proportional to the voltage, which varies sinusoidally, and since the sine of 30 is equal to one-half, the width of the green strip should be equal to one-half of the total color deflection of the beam at its point of impact if pro-per color balance is to be achieved and the television image reproduced with its true color values.

It should be noted here that the criterion just statcd is an ideal which assumes that the width of the line focus of the beam approaches zero. In practice, although small, its width is finite, and during the time when it is traversing the junction between adjacent strips it will excite the emission of two colors. In order to prevent color dilution it is therefore advisible to blank out the beam during the transition period. If the deflection of the beam is not in the proper proportion to the width of the intermediate strip, balance can still be achieved by prolonging the blanking interval, thus achieveing the balance at the expense of a diminution in the total light emitted by the screen.

It has heretofore been found necessary, in employing display tubes of the type here considered for the presentation of simultaneous or dot-sequential images, to increase the blanking periods by a very material factor over those which have been assumed to be required. Where this has not been done the color balance has been found to be seriously upset. The nature of the unbalance can be observed very clearly where the tube is used to display black-and-white images, with equal beam intensity during all three epochs of each color half-cycle. When this is done it is found that green tends to predominate strongly in the center portion of the picture field, or, alternatively, that the edges of the field display light which is definitely magenta or purplish. Unless the blanking periods have been made excessive color balance has been achieved only at the center of the screen, the ends of the screen, or in two regions located somewhere between the center and the,

ends.

The objects of this invention are to avoid the difliculties above enumerated; i. e., to provide a display surface for tubes of the character described with which color balance may be substantially maintained over the entire surface of the display screen irrespective of the system of color transmission employed; to provide a display screen which permits the presentation of true, color values with minimum periods of blanking of the beam; to provide a display surface for use with a single gun televsion tube and hence avoid all problems of registration as, between various components of a color television image; to provide a color tube having a display screen which will deliver maximum light intensity for a given electron current in the cathode-ray beam; to provide a display screen having the above stated characteristics which can be manufactured economically and sold at a reasonable price, and especially to provide a tube which will display blackand-white images of uniform tone throughout the screen without the necessity of gating the signal.

Broadly considered, the screen of the present invention corresponds to that described above, but is characterized by the fact that the strips which are electro-optically centered midway between adjacent electrodes are Wider at their ends than in their central portions. Preferably it is strips which emit green light which are shaped in the manner described, although this is for reasons apart from the present invention and color balance can be achieved regardless of which color is emitted by the phosphor so disposed. The strips which are electro-optically centered directly behind the electrodes of the grid are correspondingly narrower at their ends and wider in their middle portions. The electrodes and the strips of phosphor may be disposed either in the direction of the shorter or the longer dimension of the screen and preferably the widths of the strips at their ends are substantially uniform. Where the angle through which the cathode-ray beam is deflected in scanning the field is moderate, the outward flare of the intermediate strips from their centers to their ends may be made rectilinear, and such a conformation of the strips will give improved results with any degree of scanning deflection; preferably, however, the sides of the strips are curved, so that the outward flare of the intermediate strips increases towards their ends, or the curve may be approximated by a succession of straight lines.

The reason for the construction described and the criteria whereby best constructional parameters may be ascertained for the practice of this invention will be more fully set forth in the detailed description which follows of a preferred embodiment thereof, this description referring to the accompanying drawings wherein:

Fig. l is a schematic diagram, in longitudinal section, of a cathode-ray tube embodying the instant invention.

Fig. 2 is a face view of the central portion of a display screen embodying the invention, together with the color control electrodes controlling the portion of the screen shown, thewidths of the strips of various phosphors comprising the screen being very greatly exaggerated in comparison with their lengths.

Fig. 3 is a fragmentary cross section through the display screen and the grid electrodes, in a plane perpenducular to the latter.

' Fig. 4 is a series of graphs illustrating the optimum widths of the intermediate strips of phosphor in terms of the percentage difference in the widths of these strips from the minimum width of the strip crossing the center of the screen.

Fig. 5 is a schematic showing of the form of the display surface, illustrating the directions in which the variation of intermediate strip-width is plotted in the various graphs of Fig. 4.

The tube schematically shown in Fig. l is conventional except for the color control and display surfaces. It comprises the usual evacuated envelope 1, generally of I funnel shape, with the window 2 through which the dis- (indicated by the dotted line 11) with respect to the axis of the tube. In the discussion which follows it will he assumed that the path of the beam from a center of deflection indicated by point 13 is straight, so that its angle of incidence at the color control structure adjacent the viewing screen 5 is equal to the angle 6 included be tween the beam and the tube axis. T his assumption simplifies the description but it is the angle of incidence which is actually important.

.The color control grid is mounted closely adjacent to the screen 5. It comprises two sets of electrodes, designated respectively as 15 and 15. In the present instance these electrodes are Wires, tightly stretched on a suitable supporting frame, not shown here since structure of the grid is not a feature of this invention. This wires 15 are interconnected to a common lead 17 which is brought out through a suitable seal in the walls of the envelope 1. The wires 15' are similarly connected to a lead 17. In a tube as actually constructed the total number of wires used will be from 400 to 500 or more, one-half being connected in each set. The spacing between adjacent wires will therefore be of the order of magnitude of one picture g element as displayed upon the tube. In the example described these wires are so disposed that they run vertically when the tube is in normal viewing positon, but this is not an essential feature of the invention; other factors may make a horizontal disposition preferable.

The screen comprises a transparent base plate 19, on which is deposited a layer 21 of phosphors, and, in the tube shown, the phosphor layer is covered by a thin, electron-permeable layer 23 of conducting material, preferably aluminum. The connection 25 is brought out from the'layer 23 through the walls of the tube.

The layer 21 of phosphors is composed of strips of different phosphors, emissive, upon impact by the electron beam, of light of the different component colors which additively produce white. For convenience in description the phosphors will hereinafter'be referred to as being of the color of the light as emitted, although, in general, when viewed by reflected light, all will appear very nearly white.

The arrangement of the phosphors is shown in greatly exaggerated form in Fig. 2. This exaggeration is necessary because the actual shape of the strips could not be made apparent in a patent drawing. Thus, assuming that the particular tube shown has a 20-inch screen, i. e., a viewing area measuring 20 inches across its longest diagonal, and has an aspect ratio of 3:4, with the grid wires disposed vertically, the length of the strips will be 12 inches. In accordance with present transmission standards the theoretical number of elements per line actually displayed is about 435, so there will be this number of inter-electrode spaces across the 16 inch width of the screen, and the number of phosphor strips will be double this figure or 870. The average width of each strip in this instance will be less than 20 mils, making the ratio of length to average width over 600:1. To show strips of this aspect ratio is obviously impractical,.and therefore the width of those shown in Fig. 2 is exaggerated in comparison with their length. Since this invention is concerned with the ratio of the width of each strip at its center to its width at its ends the actual aspect ratio illustrated is unimportant as long as the fact of the exaggeration of the showing is kept in mind.

In Fig. 2 the lines 15 and 15 represent the electrodes of the sets similarly marked in Fig. 1. As has been shown in various copending applications of the present inventor, e. g., application Serial No. 265,366, now United States Patent No. 2,669,675, dated February'16, 1954, if the average potential of the two sets of electrodes with respect to the cathode of the electron gun be maintained at approximately 25% of the potential between the film 23 and the cathode, the interspace between each pair of grid electrodes becomes the aperture of a cylindrical electron lens which converges the beam from :the gun to a focus at the plane of impact with the screen. Phosphor strips 27 are electro-optically centered under the center of each aperture formed by the grid. The United States patent above identified defines the term electro-optic-ally centered and describes the relative placement of focusing electrodes and strips required to accomplish it. Briefly, the term means that when no potential diiference is applied between the electrodes 15 and 15' and the beam is so deflected that it is centered on the aperture between such a pair of electrodes, the focal point will be centered upon the strip in question. If the screen be viewed through the grid with the eye at the center of deflection, strips 27 will not appear to be alined with the centers of the aperture, since the focusing field bends the beam passing through the grid toward the screen to .a degree which depends upon the angle of deflection. The center of the aperture is therefore neither perpendicularly in 'front of the centers of the strips throughout the field nor are they optically centered with respect thereto. The

amount of relative displacement is fully discussed in the copending application mentioned. The departure from rectilinearity, with respect to any one of the component electronlenses is very slight, and except for" the general description here given it appears unnecessary to discuss the efiect further in this context, although certain factors which enter into the problem of electro-optical centering are also involved in the present case and to this extent will be referred to hereinafter.

The strips 27 are characterized by the fact that they I are materially broader at their ends than they are at their central portions. Between each pair of strips 27 there is a strip of one of the other two phosphors, strips 29, of the red phosphor, being electro-optically centered'under each of the electrodes 15 and strips 31, of the blue phosphor, so centered under each of the electrodes 15', so that, considered in order in the direction of scanning deflection of the beam, the colorcycle is red, green, blue, green, red, etc. termined by that of the strips 27, their shape being such that they are contiguous to the intermediate strips 27 throughout their lengths.

The sensitivity of the beam, to deflection by electric fields established between the two sets of electrodes of the grid, varies with its angle of incidence to the grid, being greatest at maximum deflection and least when its angle of incidence 0 is zero. The color-control deflection is always in a plane normal to the grid wires, and the variation in deflection sensitivity varies with both the angle a the component of the angle of incidence 0 parallel to the grid wires, and the angle 5, perpendicular thereto. The angles are related by the equation:

: 2D sin 0 cos 6+Vml 1 where d is the displacement mentioned, D the spacing between grid and screen, V2 the grid-to-screen voltage and V1 the cathode-to-grid voltage. This displacement is small in comparison with the size of the screen and can be neglected in so far as it afl'eots geometrical distortion of the image. It is material, however, in comparison with the width of the phosphor strips which constitute the phosphor groups or color-cells and which must be electro-optically alined with the grid apertures. It amounts to the combined width of several such color cells at the edgesof the screen. The color displayed by the screen depends on such alinement, but since the component of the displacement in the dimension parallel to the grid electrodes has no eflect on the color displayed only the component B of the angle of deflection need be considered. This component can be expressed by the equation: I

: 2D sin 0 tan 6: 2D tan B I we /eos 0+V2/V1 tan 6 1+\/1+K sec 0 where K: V2/ V1.

' B by a small incremental angle A19 whose sine is propor- The form ofthe strips 29 and 31 is de- The angles of deflec-- When voltage is applied between alternate grid wires, the wires are charged oppositely, creating an electric field which imparts to the beam a velocity vh perpendicular to the 13 plane component Vs of the total velocity v at the grid. Vh is proportional to the time spent by the beam in traversing the deflection field and can be written:

6 1 4 n 41. v1. m

The change in beam angle in the ,6 plane due to this grid deflection is:

3 l m. a?

The change in {3 plane component of beam displacement at the screen due to grid deflection is:

The grid deflection is proportional to the electric charge on grid wires produced by deflection voltage VI) and is inversely proportional to the [3 plane component of beam velocity, Vs, at the grid. Therefore cos 6 m cos B (S) q: /2C VD where C is the capacity between adjacent electrodes per cm. length of electrode of one polarity. Substituting:

y K g tan B cos B V; p Jim/V1 I VE/K1 cos 0 1 cos 6 eos 0 Note that this equation is applicable to any shape of linear electrode. The only effect of changing electrode shape is to change capacity and deflection voltage in inverse ratio, the charging current remaining constant for a given frequency of color switching.

As either 0 or B increases, the grid deflection increases. For a grid of round wires of diameter w and center-tocenter spacing s,

C =-1Z1L esu 4 me The ratio Vn/ V1 is an operating parameter which may be chosen as desired, and is uniform throughout the target area. If the spacing of the grid wires is also uniform the first term on the right-hand side of the equation becomes a constant. It is convenient to treat it as such, since as is shown in the copending applications cited, variation in grid wire spacing (where it is resorted to in order to correct for beam refraction) is always small, is preferably accomplished in steps, and is in any event a minor factor in the equation.

The quantity which is actually desired for the purposes of the present invention is the relative deflection sensitivity Sr, which is the ratio of the sensitivity at any point of the target to that at the center, where 0:0. If the electrode spacing is constant,

In the graphs of Fig. 4 values of Sr are plotted against tan a for various values of B, and against tan ,B:tan 0 where a:0. Curve 33 shows the relative sensitivity along the vertical axis of the screen, where tan 5:0. Curve 35 is plotted for tan 5:0.58; this is the edge of the screen for a 72 tube with the grid electrodes running vertically, wherein the maximum value of 0:36". Curve 37 shows the relative sensitivity at the edge of the screen for a tube, where tan fi :0.80. Curve 39 shows the variation of sensitivity with deflection along the horizontal axis of the screen, where a:0.

As these curves show, there is an advantage in using vertical strips and wires as far as minimum difference in deflection sensitivity is concerned; with a 90 tube there is about 43% over-all variation if the larger component of deflection is parallel to the grid electrodes as compared to an over-all variation of 25% if the larger component is transverse to the electrodes.

From an observers standpoint, however, there are advantages in making the grid electrodes horizontal, parallel to the scanning lines, since this improves the apparent definition and is less likely to develop moir effects. In many cases the advantages of horizontal grid electrodes become controlling; where such is the case the use of the invention is even more important than where the wires and phosphor strips run vertically, since the difference in relative sensitivity is greater.

The location indicated by the reference character 41, which is substantially at the point of intersection of the sensitivity curves, marks the maximum relative sensitivity with a 90 tube, substantially constant along the edges of the screen. Points 43 and d3 indicate maximum sensitivity for deflection along the horizontal screen axis for the same tube. Points 45 and 47 indicate the loci of maximum sensitivity for vertical scanning angles, on the vertical axis and at the corners of the screen respectively for a 72 tube while points 49 and 49' indicate maximum sensitivity for deflection along the horizontal screen axis.

The manner in which curves 33, 35 and 37 are plotted exaggerates their curvature over and above the showing of Fig. 2, since the axis of ordinates is on a scale several orders of magnitude greater than that of the axis of abscissa. Even so, it can be seen that percentage-wise their curvature is small, even with the 90 tube. The total width of the individual phosphor strips averages only from 15 to 17 mils, depending on the screen size, and with the methods of printing the screen available to date the tolerance that must be allowed for printing inaccuracies is in the neighborhood of 0.5 to 1 mil. This is from about 3% to about 6% of the strip width.

To plot the outline of each strip 27 separately, as would be required to obtain theoretical exactness, is therefore neither necessary nor profitable. The eye is relatively insensitive to small variations in color balance, even in black-and-white, although larger variations become highly visible. Therefore approximations to the theoretical form of strips which do not exceed the printing tolerance in the errors involved, are justifiable.

The widths of the strips 27 should vary with variations in the angle or in accordance with the values plotted in Fig. 4. It will be seen, however, that for a 72 tube the graph 51, comprising two straight lines, nowhere departs from either curve 33 or curve 35 by more than 3.5%. Therefore a very satisfactory approach to the optimum shape of strip can be achieved by giving the edges of the strips a straight-line contour, all of the strips 27 being of the same shape and dimensions, it being understood that the widths of the strips vary with length in proportion to the ratio of the values plotted to its minimum value. The errors will be least in the zones where ,8 is about 22% or about of the total horizontal deflection. Alternatively, strips of two, three or more different contours can be used in successive zones, from the center outward. This will be necessary in any case with tubes using wider deflection angles if the suggested tolerances are tobe maintained. It

should be obvious that even a very rough approximation to the theoretical form will give greatly improved results, either as to color balance, reduced blanking. periods or both.

The above concerns the relative widths of the phosphor strips but not their relationship to the strips 29 and 31. With sinusoidal color-deflecting potentials the excursion of the beam onto these flanking strips will be /2 that from the center of the strip 27 to its edge. Therefore if the deflecting potentials used are such that the spot is centered under elecrtode 15 or 15 at the points of maximum deflection and maximum sensitivity (neglecting, for the moment, the size of the spot), the widths of all of the strips should be equal'at these points; i. e., at the ends of the strips. If, in order to obtain maximum luminance at the expense of some color definition at the upper and lower edges of the screen, the beam is deflected under the electrodes, and to the extreme edges of the strips 29 and 31, so that'at the edges of the field the adjacent color cells overlap, the strips 27 and 31 may be made only slightly more than l/z the width of strips 29 at their ends.

The actual size of the spot that can be produced is also a factor. Experience has shown that the focusing potentials which theoretically should reduce the spot to a geometrical line, without width, actually give a spot width of about 3 mils, owing to scattering of electrons in the metal layer and the phosphor, scattering of light in the phosphor, and to a minorextent, electron lens aberrations.

The deflection of the beam electrons which causes the focusing varies in accordance with an equation of substantially the same form as that for color deflection relative sensitivity, unity relative sensitivity at the center of the screen corresponding to minimum width of spot at the same point for the same value of K=3 and the beam being over focused by the same factor as the sensitivity to deflection is increased. Thus at the point 47 the convergence is about 1.165, which means that the theoretical width of the spot is 16.5% of the width of the aperture between the electrodes. Ir" the spacing of the grid wires is 30 mils on centers, and the wires are 6 mils in diameter, this gives a theoretical spot size of .165 X24 mils or about 4 mils. To this must be added the 3 mil minimum, so that in the 72 tube the spot size will vary between 3 mils atthe center and 7 mils at the corners of the screen. With a 90 tube the maximum spot width, with the same electrode spacing, would be 9 mils if the maximum constriction is at the center of the screen.

It is possibe to vary the value of K slightly so that the finest focus is in an intermediate zone, the beam being sightly underfocused at the center and overfocused to a smaller degree at the edges. By this expedient the maximum width of spot may be made about mils for a 72 tube and 6 mils for a 90 tube. If this is to be done it is the better practice to compute both the correction for refraction of the beam and the curves for deflection sensitivity in accordance with the modified value of the quantity K, although the ditference from the values where K=3 will be small in any event.

Even a 3 mil spot represents a material fraction of the width of the color strips. Since the spot should be blanked as soon as its edge touches the junction between strips its effective dwell on the strips 27 is not the, time required to traverse its center thereacross but the time required to traverse its center across astrip which is narrower than the actual strip by the Width of the spot.

The desideratum is that the dwell on the strips 27 shall be uniform throughout the picture field. Therefore the optimum method of employing the invention is to compute the proportional widths of the strips 27 on the basis of Equation 11 or the curves of Fig. 4 and then add to the widths thus computed the width of the spot. The width added may be either graduated to accord with the actual width at various angles of deflection, or it may be the maximum spot width which is added throughout the entire length ofthe strip. Iii the latter case if the blanking period is adjusted to prevent the spot'falling on two phosphors simultaneously at the positions of maximum spot size no overlap will occur at any part of the field.

With the phosphor strips thus formed, the dwell on the strips 27 will be uniform throughout the picture field to within the degree of approximation to which the strip widths approach their theoretical values. This will be so regardless of the amplitude of color deflection. If this amplitude is too low there will be some color dilution and the color balance will tend toward the green, if the amplitude is too great there will again be some dilution and the color balancewill tend toward the purple. By proper adjustment of deflection amplitude an accurate color balance and minimum blanking period can be attained.

The copending applications mentioned show that with scanning deflections asgreat as those here considered it is necessary to vary the ratio of strip width to electrode spacing with varying values of both the angles a and ,8 to maintain the grid apertures and the centers of the color cells in electro-optical alinement, and that this may be done either by maintaining the widths of the color cells constant throughout the screen and varying the electrode spacing or vice versa, or by a combination of both expedients. In either case the practical solution is to accomplish the correction in steps.

If it is the electrode spacing which is varied the result is a partial reduction of the variation in deflection sensitivity. The first term of Equation 11 is no longer a constant, but must be recomputed for each zone in which the spacing is varied. If the width of the color cells is varied the variation mustbe confined to the strips 29 and 31. I a I Thescreen in either case may be constructed by machining metal masks to the required contour and moving them, step-by-step, across a surface from which a master pattern is to be prepared, for example a silk screen which is to form a wax stencil, or a photographic plate which is exposed line-by-line.

A more elegant method of preparing the screen is to construct a target using a grid of the same form as is to be empoyed in the commercial tubes, but to replace the screen with either a photographic plate or a monochrome fluorescent screen. This target is placed in a demountable tube having the same structural parameters as those in which the targets are to be used. Focusing voltage and scanning deflection ratios are adjusted to optimum values. The color deflection voltage, however, is reduced so that the excursion of the spot is limited to the desired width of the strips 27 at the positions of maximum deflection sensitivity.

If aphotographic plate is used as the target the beam itself accomplishes the exposure. If a fluorescent screen is used it is photographed in the usual manner. In the latter event great care must be exercised to ensure that aberrations due to the tube window and the screen itself do not cause distortion. In either case a master negative is produced of the strips 27, including all corrections for refraction, deflection sensitivity and spot size. Duplicate negatives on glass provide patterns from which the stencils or other printing media for the strips 29 and 31 may be prepared by blanking out alternate white lines.

Each of the modifications of this method has its advantages. The use of the photographic target prevents aberrations; the fluorescent screen target permits direct observation for the control of spot size, deflection amplitude, etc.,,but either modification is capable of producing good screen if adequate care is exercised.

In order to obtain the maximum benefit from the invention it is obvious that a fully corrected target, such as is produced by the photographic methods above described, should be used. Very considerable advantage is gain, however, from the more approximate corrections and these advantages are considerably greater than the percentage figures on deflection sensitivity would indicate. This is best brought out by consideration of the color unbalance produced in black-and-white where the invention is not employed. Assuming a 72 tube, with the deflection adjusted to produce color balance at the ends of the strips, the total amplitude of deflection may be taken as 30 mils, in which case the effective dwell on the green is 60 out of each half cycle. At the center of the screen the amplitude is reduced to 26 mils, increasing the dwell on the green to nearly 70 out of the half cycle and reducing the dwell on each or the others to 55. The 15% difference in deflection sensitivity increases the green by a factor of 1.20 and reduces the other two colors by 9% each. From another point of View, the green, instead of being equal in value to each of the other two color components, is about 27% greater, which is enough to be unpleasant to the viewer.

if the correction of strip shape is made to within plusor-minus' 3%, as has been here suggested, the width of the green strips at the center of the field will be reduced from 15 mils to somewhere between 12.6 mils and 13.4 mils, and nominal value being 13 mils. Taking the maximum value of 13.4 mils, the dwell on the green will be 62 and that on the other two 59 each. The green is therefore only 5% greater than the other two, a 51l reduction in unbalance. Since the small amount of unbalance varies gradually over the screen it is practically unnoticeable. Therefore, although the complete correction is to be preferred even rougher approximations than those here suggested will give materially better resuits than the use of straight phosphor strips.

Repeated reference has been made herein to the ap proximations and tolerances involved in proportioning the screen with respect to the grid in order to produce eiectro-optical alinement of the grid apertures and color cells, and to the fact that the variations in spacings may be made in step. As a result of this method of correction there are cumulative errors of alinement in certain por tions of the screen, and it is of int rest to note what the effect of these errors may be on the present invention.

As viewed by the spectator the effect is practically nil. Considering, for example, a portion of the screen where as a result of such cumulative error of the maximum assumed amount a green strip is off center by 1 mil toward the red. The amount of red exhibited by this cell will therefore be decreased and the amount of blue increased by very nearly a like amount; at the center of the screen, with a 13 mil width of the green strip at this point, the red will be reduced from 60 electrical degrees to 55, while the dwell on the blue will be increased from s 60 to 65.2 in each half cycle. The dwell on the green will be reduced by 02 or 0.13%.

In the two adjacent cells, however, the conditions will be reserved, the dwell on the blue being reduced while that on the red is increased, the dwell on the green remaining the same within observable error, since the variation in theoretical spacing ratio in adjacent cells is of the order of one-millionth of an inch. All this, it is to be remembered, refers to ungated black and White signals.

To present the proper illusion of color the screen must be viewed from a suflicient distance so that the spots of illumination from adjacent color cells merge. The total average illumination from adjacent color cells remains sensibly the same and the only effect of the inaccuracy, as far as color balance is concerned is a displacement of the red and blue elements from their proper positions by one mil, which is undetectable.

In the reproduction of color pictures, where the beam is intended to be blanked during its transition from stripto-strip, the effect may be somewhat greater if the blanking period is reduced to its theoretical minimum. The blanking pulses will be displaced from their theoretically proper positions by electrical degrees and it might be expected that this would have serious effects. Such effects are reduced, however, by two factors; first, the spot is not of constant intensity throughout its cross section, but is brighter in the center, and second, the pulses used for blanking the spot have a finite rise time. As a consequence, when the spot starts to overlap a junction before blanking is complete, because of an inaccuracy of alinement, it is the less-intense edge which first excites the undesired color and as the spot continues to move toward that color it is fading rapidly. The intensity of the desaturating or diluting light is therefore much lower than mere figures as to spot size would indicate. The unblanking of the spot as it reaches the next color results in no dilution, since the spot will be wholly on that color when unblanking occurs. Similarly, if the spot is blanked prematurely, it will be only the edge which remains on the wrong color when unblanking takes place, and again the intensity will be low.

in the case of the 1 mil displacement above considered for illustrative purposes, as the beam crosses the junction from red to green, blanking will be complete before the spot reaches the junction, but will be unblanked before fully off the green, giving a slight dilution of the latter. There will also be an excess of green as the spot crosses the junction in the reverse direction. As it approaches the blue there will be an overlap giving an excess of blue, resulting in a cyan tinge to the color perceived from that particular cell. In the adjoining cells the situation as regards rec and blue will be interchanged, and the off-color tinge will be toward the orange.

As it works out, if the adjustment of the amplitude of the color deflection is such as to give a true balance when the proper spatial relations exist, the proportions of red, green and blue that are emitted as a result of misalinement are such as to produce a substantially pure white of low intensity when merged by the eyes of the observer. That such merging will actually occur is insured by the fact that the eye resolves color to a much more limited degree than it does brightness. That this is the case is evidenced by the fact that the NTSC proposed standards, adopted after prolonged tests and tentatively approved by the Federal Communications Commission, provide for the transmission of color information with only /8 to A of the detail of the brightness information. Each color cell is of approximately the width of 1 picture element of maximum detail; the eye needs merge only the color of 2 such cells to produce the sensation of white, whereas the minimum to produce separate color sensations would be at least 4.

The effect of the white light thus fortuitously produced is merely to reduce slightly the saturation of pure color without a change .of hue. Saturated colors are substantially unknown in nature and variations in the degree of apparent saturation observed occur with change of angles of observation and of illumination of a colored surface. The eye therefore accepts such variations as natural and usually does not even recognize that they exist. Misalinements of the order mentioned are substantially without visible effect in connection with the present in vention, although where the invention is not employed, misalinement can accentuate the difiiculties it is devised to overcome.

The fact that the effect of slight overlaps is so small can be taken advantage of in tubes using wide scanning deflection angles to reduce the blanking time allotted because of the size of the spot at the edges of the picture field. Where the spot is large it is also diffuse and its intrinsic brilliancy is reduced and such reduction is effective at the dimmer edges well as the brighter center. in medting the correction for spot size in the contour of the phosphor strips it may therefore be desirable to assume a width somewhat less than the maximum, and hence allow some overlap, at the edges of the picture field, in the transition from phosphor to phosphor. Since vision is usually fixed on the center of the field and color perception is greatest at the center of vision, slight desaturation is less noticeable at the edges of the field than in the center;

13 .Reference has been made above .to the Patent No. 2,669,675 to the same inventor, defining the relationships between phosphor strips and grid wires which are required in order to attain substantial electro-optical centering of the strips.

It is there shown that with tubes using scanning deflections as large as those here contemplated'there must be a variation, from the center of the screen outward to the edges, in the ratio of the pitch of the grid electrodes to the widths of the phosphor groups or color cells and that for substantially complete correction of centering errors the color cells should be wider at their centers than at the edges of the screen if the, grid electrodes are straight. Since the color cells, in this case, are wider at their centers than at their ends, and since the width of each color cell is the sum of the widths of the three strips which comprise'it this may raise some question as to whether the two ii'iventions are compatible. 7 I

That they are compatible will be evident from the considerations which follow: To provide compensation for the barrel distortion in tubes using straight grid electrodes it is the phosphor groups as a whole, each including portions at least of three phosphor strips, which are wider at their centers than their ends. In a 72 tube of the character described herein the additional group width required on the axisof'the screen is on the average almost exactly 0.1%.. With some methods of manufacture as there described each strip will be widenedin like proportion. With'an average width of strip of 15 .mils this involves an average increase of 15 millionths of an inch at the center of the strips, disregarding the deflection sensitivity correction described herein. Cumulatively the outermost strips are bowed outwardly by about one-h alf one strip width.

Applying the present invention, with contiguous strips of equal widths at their ends (say 15 mils), the center or green strips will be about 12 mils wide at their centers without the barrel correction and 12.012 mils wide when the correction is made, while the contiguous strips of red and blue emitting phosphors will be 17.988 mils wide at their centers. The only observable effect of employing the barrel correction together with the present invention is the slighttbowing of the outer strips of'the' phosphor pattern; thebarrel correction is so small as to be completely masked by the correction herein described as far as any individual strip is concerned. Even the outward bowing of the strips is so slight as to be almost invisible unless compared with a straight edge. Where the photo graphic method of forming the screen is employed, as described above, both the correction of color-cell spacing and of deflection sensitivity. are accomplished simultaneously. I

As is brought out in the companion application, the variations from color-cell to cell are extremely minute; smaller, on the, average, than the tolerances that can be met in commercial tube manufacture. The cell-to-cell variations, however, are cumulative. The intent of the present invention as well as that of the companion appli cation is to compensate the cumulative variations within the limits of manufacturing tolerances. To build a tube exactly conforming to the equations set' forth herein would require a degree of precision far beyond that normally attained in tube manufacture. Tubes wherein the cumulative variations intended to be compensated are less or, at least no greater than the individual manufacturing tolerances, thus substantially conforming to the equations given, can be manufactured economically in quantity using presently available. techniques.

The above considerations have been expounded in; detail to indicate the broad nature of the invention and that although its best application involves mathematical exactness its value is not limited to embodiments or operations where such exactitude is observed. The numerical examples and computations given are intended to be illustrative and not as'limitations, all intended limitations being set forth in the, claims.

What is claimed is as follows:

' 1. In combination with a cathode-ray tube for displaying television images in natural color which includes an electron gunfor directing a beam of cathode-rays toward a target area within said tube over which area said beam is adapted to be deflected in two dimensions to trace a raster and. a substantially planar color control'grid comprising two interleaved sets of linear electrodes the interspaces between which are of the order of magnitude of one elemental area of the television images to be reproduced: a display screen defining said target area comprising a base plate mounted in a plane substantially parallel and adjacent to said target area, and a coating on said display screen comprising strips of three diiferent phosphors respectively emissive on electron impact of light ofthree component colors additive to produce white, the strips of one of said phosphors being elect ve-optically centered behind the interspaces between the electrodes of said grid and being "wider at their ends than at their central portions and the strips of the other two of said phosphors being electro-optically centered respectively behind the electrodes of said two sets and forming with said first-mentioned strips a substantially continuous coating.

2. In combination with a cathode-ray tube for displaying television images in natural color which includes an electron gun for directing a beam of cathode-rays toward a target area within said tube over which area said beam is adapted to be deflected in two dimensions to trace a raster and a substantially planar color control grid comprising two' interleaved sets of linear electrodes the interspaces between which are of the order of magnitude of one elemental area of the television images to be reproduced: a display screen defining said target areacomprising a base plate mounted in a plane substantially parallel and adjacent to said target area, and a coating on said display screen comprising strips of three different phosphors respectively emissive On electron impact of light of three component colors additive to produce white, the strips of one of said phosphors being electro-optically centered behind the inters'paces between the electrodes of said grid and being wider at their ends than at their central portions andthe strips of the other two of said phosphors beingelectro-optically centered respectively'behind the electrodes of .said two sets and forming with said firstmentioned strips a substantiallycontinuous coating and the widths of the ends of contiguous strips being substantially equal.

3. In combination with a cathode-ray tube for displaying television images in natural color which includes an electron gun including an electron emitting cathode for directing a beam 'of cathode-rays toward a target area within'said tube over which area said beam is adapted to be "deflected'in'two dimensions to trace a raster and a substantiallyplanar color control grid comprising two interleaved sets of linear electrodes the interspaces between which are of the order of magnitude'of one elemental-area of the television images to'be reproduced; a display screen defining said target area comprising a base plate mounted in a plane substantially parallel and adjacent to saidtarget area, and a coating on said display screen comprising strips of three difierent phosphors respectively emissive onelectron impact of light of three component colors additive to produce white, the strips of one of said phosphors being electro-optically centered behind the interspaces between the electrodes of said grid and widths of said strips varying along the lengths thereof so that the width at any portion bears to the width of the corresponding strip atthe center of said screen substantially the ratio cos 5| 1+ /1+K tan a :l "5052 L1 /1 K see JE KK Q where 6 is the angle of incidence of the bearn'at the color control grid, [3 is the component of 9 normal to the linear electrodes comprising the grid and K is the ratio of the 15 voltages between grid and screen to that between the grid and the electron gun cathode for which said strips are electro-optically centered behind the interspaces between said electrodes, and strips of the other two of said phosphors disposed alternately in the spaces between said first mentioned strips.

4. In combination with a cathode-ray tube for displaying television images in natural color which includes an electron gun including an electron emissive cathode for directing a beam of cathode-rays toward a target area within said tube over which area said beam is adapted to be deflected in two dimensions to trace a raster and a substantially planar color control grid comprising two interleaved sets of linear electrodes the interspaces between which are of the order of magnitude of one elemental area of the television images to be reproduced; a display screen defining said target area comprising a base plate mounted in a plane substantially parallel and adjacent to said target area, and a coating on said display screen comprising strips of three different phosphors respectively emissive on electron. impact of light of three component colors additive to produce white, the strips of one of said phosphors being electro-optically centered behind the interspaces between the electrodes of said grid and increasing in width progressively from their central portions outwardly to their ends substantially by increments cos d tan? 5 1 b cos a 5r7,] -,/1 +K sec where W0 is the width at the center of the central strip minus the maximum width of the spot produced by said beam, 9 is the angle of incidence'of the beam at the grid, 5 is the component of 6 normal to the grid electrodes, and K is the ratio of the voltage between grid and screen and to that between cathode and grid for which said strips are electro-optically centered, and strips of the other two of said phosphors disposed alternately in the spaces between said first described strips.

5. The combination as set forth in claim 4 wherein the increase in the widths of said first mentioned phosphor strips as defined therein is approximately by a series of strai ht lines defining the edges of said strips.

6. The combination as set forth in claim 4 wherein the quantity K is substantially equal to 3.

7. In combination with a cathode-ray tube for displaying television images in natural color which includes an electron gun including an electron emissive cathode for directing a beam of cathode raystoward a target area within said tube over which area said beam is adapted to be deflected in two dimensions to trace .a raster and a substantially planar color control grid comprising two interleaved sets of linear electrodes the interspaces between which are of the order of magnitude of one. elemental area of the television images to be reproduced; a display screen defining said target area comprising a base plate mounted in a plane substantially parallel and adjacent to said target area, and a coating on said display screen comprising strips of three different phosphors respectively emissive on electron impact of light of three component colors additive to produce White, the strips of one of said phosphors being electro-optically centered behind the interspaces between the electrodes of said grid and increasing in width from the centers to the ends thereof substantially in proportion to the increase of said beam in sensitivity to deflection by said grid with increase in angle of incidence thereto, and strips of the other two of said phosphors disposed alternately in the spaces between said first described strips.

8. in combination with a cathode-ray tube for displaying television images in natural color which includes an electron gun including an electron emissive cathode for directing a beam of cathode rays toward a target area within said tube over which area said beam is adapted to be deflected in two dimensions to trace a raster and a 1 5 substantially planar color control grid comprising two interleaved sets of linear electrodes the interspaces between which are of the order of magnitude of one elemental area of the television images to be reproduced; a display screen defining said target area comprising a base plate mounted in a plane substantially parallel and adjacent to said target area, and a coating on said dis play screen comprising strips of three different phosphors respectively emissive on electron impact of light of three component colors additive to produce white, the strips of one of said phosphors being electro-optically centered behind the interspaces between the electrodes of said grid and increasing in width from the centers to the ends thereof substantially in proportion to the sum of the increase in sensitivity of the beam to deflection, by said grid and the variation in dimension of the focal spot with increase in angle of incidence of said beam at said grid, and strips of the other two of said phosphors disposed alternately in the spaces between said first described strips.

9. A display screen for cathode-ray tubes for producing television images in color and employing lens-grids comprising a multiplicity of generally parallel linear conductors for establishing a system of electron lenses adjacent to said display screen, comprising a supporting base, and a phosphor coating on said base consisting of generally parallel strips of three dilferent phosphors respectively emissive on electron impact of light of three component colors additive to produce white light and arranged in a repeating pattern, alternate strips being wider at the end portions than at the central portions thereof and the intervening strips being wider at the centralportions than at the ends thereof.

ll). Target structure for color television display tubes comprising a grid formed of two interleaved and mutually insulated sets of generally parallel elongated linear conductors, the conductors of each set being interconnected, a base mounted adjacent to said grid, a coating on said base consisting of three different phosphors which are respectively emissive on electron impact of light of different colors additive to produce White light, said phosphors being disposed in a repeating pattern of strips generally parallel to said conductors and the number of strips being twice the number of int rspaces between said conductors, alternate strips being Wider at the end portions than at the central portions thereof and the intervening strips being wider at the central portions than at the end portions thereof, and a conductive coating deposited on said phosphor coating.

ll. Target structure for cathode-ray tubes adapted for displaying television imagesin substantially natural color comprising a color-control and lens-grid formed of two interleaved and mutually insulated sets of generally parallelly strung elongated linear conductors, the conductors of each set being electrically interconnected, a base positioned adjacent to said grid, a coating on the case surface adjacent to said grid comprising phosphors of three different light producing characteristics forming a color triplet the colors of which are developed upon electron impact and are additive to produce white light, the phosphors being disposed on the base in a repeating pattern of strips generally parallel to the linear conductors of the grid, alternate phosphorcoated strips being wider at the end portions than at the central portions thereof and the intervening phosphor-coated strips being wider at the central portions than at the end portions thereof, the linear conductors of the grid being supported in generally juxtaposed relationship'to the phosphor coatings with the electrodes of the grid electro-optically centered relative to alternate phosphor-coated strips, and-an electrically conductive coating disposed on the surface of said phosphor coating adjacent to the grid.

lll arget structure for cathode-ray tubes adapted for di television images in substantially natural color comprising acolor-controi and lens-grid formed of two interleaved and mutually insulated sets of generally paral 17 lelly strung elongated linear conductors, the conductors of each set being electrically interconnected, a target surface supported adjacent to said grid, phospor coatings on the target surface adjacent to said grid, the said phosphor coatings being of three different light color pro ducing characteristics collectively forming color triplets which upon electron impact are light-emissive additively to produce White light, the different characteristic phosphors being disposed on the target surface in a repeating pattern of strips generally parallel to the linear conductors With alternate phosphor-coated strips being Wider at the end portions than at the central portions thereof and the intervening phosphor-coated strips being Wider at the central portions than at the end portions thereof, the linear conductors of the grid being supported in generally juxtaposed relationship to the phosphor coatings, and electro-optically centered relative to alternate phosphorcoated strips, an electrically conductive coating disposed on the surface of said phosphor coatings adjacent to the grid, and terminals toapply electrical potentials to the conductors and the electricallyconductive coating in the phosphors.

References Cited in the file of this patent UNITED STATES PATENTS 2,429,849 Somers Oct. 28, 1947 2,619,608 Rajchman Nov. 25, 1952 2,669,675 Lawrence Feb. 16, 1954 

